1. Field of the Invention
This invention relates to a hydrogen absorbing alloy powder and electrodes formed of the same. More particularly, it relates to a hydrogen absorbing alloy powder which can suitably be used to form the negative electrodes of nickel-metal hydride storage batteries, and to electrodes formed of the same. Still more particularly, it relates to a hydrogen absorbing alloy powder for use in nickel-metal hydride storage batteries having a high capacity, excellent initial characteristics and a long life.
2. Description of the Related Art
Since the discovery of hydrogen absorbing alloys capable of absorbing and releasing hydrogen, they not only have been used as hydrogen storage means, but also have been more and more widely applied to heat pumps, batteries and the like. In particular, alkaline storage batteries using a hydrogen absorbing alloy powder for the negative electrode have already been put to practical use, and various attempts have successively been made to improve the capacity and life of the batteries in view of the hydrogen absorbing alloy used therefor.
That is, with respect to the initially investigated LaNi.sub.5 alloy having a CaCu.sub.5 type crystal structure, an improvement in capacity and life has been achieved by replacing a part of La with Ce, Pr, Nd and other rare earth elements to form misch metal (Mm), or by replacing a part of Ni with a metallic element such as Al, Co or Mn.
However, when such hydrogen absorbing alloys are used to form electrodes for batteries, the capacity and life of the resulting batteries can be improved, but their initial characteristics are reduced.
Generally, the capacity tends to become higher as the La content in Mm is increased. When the La content is 100%, a maximum capacity is obtained, but the cycle life becomes extremely poor. Moreover, when a part of Ni is replaced with Co, Mn, Al or the like in order to achieve a long life and prevent passivation, the equilibrium pressure is lowered to cause a reduction in low-temperature characteristics and high rate discharge characteristics. Accordingly, in order to bring about an improvement in life, it has conventionally been proposed to use Mm in which a part of La is replaced with the other light rare earth element (Ce, Pr or Nd), decrease the Ce content of Mm (Japanese Patent Provisional Publication No. 62-223971/'87), or, on the contrary, add Ce thereto positively (Japanese Patent Provisional Publication No. 6-96766/'94). However, it has been impossible to obtain a battery having a high capacity, a long life and excellent characteristics solely by resorting to an improvement in alloy composition.
Initial characteristics are generally expressed in terms of the number of charging-discharging cycles repeated until a maximum capacity is reached, and they are considered to be higher as the number of cycles is smaller.
Usually, initial characteristics are evaluated by the capacity in the first cycle. However, a battery having low initial characteristics has the disadvantage that, when it is fabricated into a sealed type one, the balance between the positive and negative electrodes is lost to cause a reduction in the capacity and life of the battery.
In order to overcome the above-described disadvantages, it has been conventional practice to treat a hydrogen absorbing alloy with an aqueous alkaline solution or inorganic acid. However, the alkali treatment has been disadvantageous in that treating conditions such as a high concentration and a high temperature are required and it is difficult to wash the treated hydrogen absorbing alloy with water. In addition, the hydrogen absorbing alloy treated by the alkaline solution or by inorganic acid undesirably undergoes a compositional change because the alloy is subject not only to surface oxidation during treatment but also to oxidation during drying and storage. Hence, the difficulty in handleability and safety makes a procedure for forming negative electrodes for batteries more complicated.